I'm going to reinstall the magneto on the 1943 M20 and had a question regarding the shim that goes between the base of the magneto and the engine. The old shim is just about gone, worn and 3 of the 4 holes are not there anymore. I have a couple of new shims. What measurement needs to be taken to determine if one or two shims are needed? I've looked through my repair data and riders manual but could not find anything.
I don't think there are shims as standard. Probably a workshop mod for an engine that has been run for a long period with a loose mag strap and it's worn the crankcase mag platform, and shimmed as a way to counteract the wear. The only thing you can do is add shims underneath to create a small amount of lash between the mag pinion and the idler gear. The teeth should be properly engaged with a tiny bit of lash so that the teeth are not bottoming out on each other. The mag should also end up at 90 degrees to the crankcase and not tipped. Ron
I was going to ask a simular question my mag pinion and idler pinion (I think it's the one Ron describes ) don't seem to mesh to a great depth.Teeth only interlock about half to three quarters along there length . (Hope this makes sense) so my question is is that ok and if not what's best cure cheers Andrew
Same thing applies Andrew. You should be able to slacken the clamp and slide shims underneath without losing your timing. You can use old feeler gauges but better to have some brass shim-stock in various thicknesses and cut to as big an area size as you can. It's trial and error.
A correction to my post regarding Andrews query. I wasn't concentrating and now realise your mag is riding a touch high. It could be that the locating pegs in the bottom of your mag are too long and fowling the bottoms of the recesses in the platform. Sometimes the wrong pegs are used
and they need flats filed into them so that they locate in their groves properly. Ron
Thanks Ron, it was interesting that it is not mentioned in the manuals, so probably just a repair on a well worn machine! I'll also keep an eye on the alignment.
My M21 had shims installed new from the factory. Base of magdyno and engine platform have no wear at all. Without shimming the gears have no clearance.
BSA service sheet 305 states shims are fitted if necessary when the engine is built and may be required if a different magdyno is fitted. No mention is made as to what the correct gear backlash should be other than if excessive it will be too noisy and if insufficient it will whine.
My gap seems alittle to big between mag and next pinion Henk I think Ron could be right and the mag is sitting a little high.I'm slowly learning its a case of slowly correcting 50 years of Billy bodgers
